As surface-accessible deposits of hydrocarbons and minerals are exploited, underwater deposits are becoming increasingly important. In oil and gas, for example, technology to locate and exploit hydrocarbon deposits is relatively mature. In mineralized deposits, however, such technology is in its infancy.
Common techniques for locating subterranean deposits of hydrocarbons and minerals include seismic analysis, Magneto Tellurics (“MT”), and Direct Current (“DC”) resistivity measurements. In seismic analysis, seismic waves are emitted into subsurface formations and the reflected or diffracted waves detected. Acoustic waves reflect off interfaces between different types of rocks with dissimilar seismic impedances. MT is a low-frequency electromagnetic induction method for determining the subsurface distribution of electrical resistivity (ρ), or its inverse electrical conductivity (σ), using measurements of naturally occurring magnetic and electric fields. In DC resistivity, current is flowed through a formation and the resultant voltage drops across spaced-apart electrodes or magnetic fields produced by the current measured.
Several methods are now used to measure the resistivity structure of the ocean bottom. In one method, coils are used as both the transmitter and receiver. This system is analogous to systems used on land and in airborne prospecting systems (for example in helicopter electromagnetic prospecting systems). In another method, an electric current dipole is used as a transmitter, and an e-field dipole is used as a receiver. This system is analogous to prospecting systems used on land, for example induced polarization prospecting systems. In another method, a controlled electric current dipole is moved through the water and E-field and H-field measurements are made by an array of E-field and H-field sensors positioned at fixed sites on the ocean bottom. This is called CSEM (Controlled Source ElectroMagnetics) and is analogous to CSAMT (Controlled Source Audio Magneto Tellurics) surveys used on land. In another method, pipe tracking EM systems primarily use colinear coils (normally two horizontal coils) for transmitting and receiving and detect a perturbed secondary magnetic field in phase or amplitude, or transmit an alternating current through a direct electrical connection into a pipe or cable and then detect the resulting alternating magnetic field using a receiver coil. These systems can be difficult to deploy underwater. In normal practice, they are used to measure large scale conductivity structures and lack the small footprint needed for small scale conductive or resistive features on the ocean bottom.